Purified IgG, F(ab')2 fragments, and Fab' fragments of monocloal B6.2 have been generated. The IgG and its fragments were radiolabeled with I-125 without loss of immunoreactivity and were injected into athymic mice bearing human mammary tumor transplants. The radiolabeled antibody localized in the tumor within 24 hours with tumor to tissue ratios rising over a 96-hour period. The F(ab')2 was better than the IgG and gave tumor to liver and spleen ratios of 15 to 20:1, and tumor to muscle and brain ratios of 50 to 110:1. No localization was observed in mice bearing human melanomas or with radiolabeled normal murine IgG in human mammary tumor bearing mice. The ability of the radiolabeled antibody to localize in mammary tumors was sufficient to give high quality gamma scans of tumor bearing mice. Several monoclonal antibodies are now being labeled with other radioactive isotopes that may be more appropriate for clinial studies.